Process and apparatus for producing sheet film from tubular thermoplastic resin film

Abstract

A sheet film is produced from a tubular thermoplastic resin film by cooling a molten tubular film of crystalline thermoplastic resin, for example, polypropylene, extruded downward from an annular die in direct contact simultaneously with cooling liquids on the outside and inside of the film, the cooling liquid on the outside being in an outside cooling liquid tank and in contact with the outside of the film and the cooling liquid on the inside flowing down along the surface of a mandrel provided through the outside cooling liquid tank and in contact with the inside of the film, a liquid level of the cooling liquid on the inside being kept higher by 5-15 mm than that of the cooling liquid on the outside; cutting open length wise one or two side ends of the film just before being flattened and folded by nip rolls thereby to discharge the cooling liquid from the inside; and removing the liquid attached to the surfaces of the resulting sheet films withdrawn by the nip rolls promptly. The sheet films having a good transparency, good flatness and less uneveness in thickness can be thereby produced at a high speed.

What is claimed is: 
     
       1. A process for producing a sheet film from a tubular thermoplastic resin film which comprises: a. extruding a molten tubular film of crystalline thermoplastic resin downwardly through an annular die,   b. passing the outside of said downwardly extruded tubular film into contact with a body of a cooling liquid,   c. introducing cooling liquid into the interior of said downwardly extruded tubular film so that a body of cooling liquid is formed in the interior of said tubular film opposite to the body of cooling liquid contacting the exterior of said tubular film,   d. maintaining the level of the body of cooling liquid on the inside of said tubular film 5 - 15 mm higher than that of the level of the body of cooling liquid on the exterior of the tubular film,   e. flattening the cooled film,   f. providing a lengthwise cut in said tubular film immediately prior to step (e) and permitting the liquid which was used to cool the inside the tubular film to flow out, and   g. removing essentially all liquid from the surfaces of the tubular film after it has been cut and flattened.   
     
     
       2. A process according to claim 1 wherein the thermoplastic resin film is a homopolymer or a copolymer of propylene. 
     
     
       3. A process according to claim 11 wherein the molten tubular film is cooled by contact with forced air prior to step (b). 
     
     
       4. A process according to claim 11 wherein the film after step (e) is subjected to mechanical surface-leveling treatment by passing through a series of several rolls under a tension of less than the elastic limit of the film, heated to a softened state, and then cooled down to room temperature. 
     
     
       5. An apparatus for producing a sheet film which comprises in combination. a. an annular die associated with an extruder which is adapted to produce a molten tubular resin film,   b. an annular ring positioned below said annular die which is adapted to cool the exterior surface of a molten tubular film extruded through said annular die,   c. a first reservoir for cooling liquid positioned below said annular air ring and having an interior perimeter that is adapted to surround said tubular film so that the cooling liquid will be in direct contact with the exterior surface of the tubular film,   d. a second reservoir for cooling liquid also positioned below said annular air ring, whose exterior perimeter is within the interior perimeter of said first reservoir and which is adapted to directly contact the interior surface of the tubular film,   e. slitting means positioned below said first and second reservoirs which slitting means is adapted to make a lengthwise opening in at last one side of the tubular film,   f. means for flattening said tube after it has been slit, and   g. means for removing residual liquid from said film after it has been slit,   h. said second reservoir for cooling liquid containing liquid height adjustment means which is adapted to maintain the level of liquid in said second reservoir higher than said first reservoir.   
     
     
       6. An apparatus according to claim 5 wherein said first reservoir includes a cooling liquid supply pipe, an overflow pipe and an elastic material adjacent the bottom of the reservoir which is adapted to form a seal between the bottom of the reservoir and the tubular film. 
     
     
       7. An apparatus according to claim 5, wherein the slitting means is a disc slitter. 
     
     
       8. An apparatus according to claim 5 wherein the means for removing residual liquid comprises a compressed air blowing device for blowing air onto both the inside and outside of the film, and vacuum suction rolls having openings of a few millimeters in diameter on the roll surface which are connected to a vacuum pump.